The present invention relates, in general, to rectifier circuit and, more particularly, to synchronous rectifier circuits.
Electronic devices such as microprocessors now operate with lower power supply ranges and higher currents than in the past. Synchronous rectifiers provide an efficient way of generating the required low operating voltages and high currents for the electronic devices. Synchronous rectifier circuits are used in forward converters, flyback converters, buck converters, push-pull converters, and half-bridge converters, among others. In forward switching power supply circuits employing synchronous rectifiers, the secondary side diodes are replaced by power transistors to obtain a lower on-state voltage drop. The synchronous rectifier uses N-channel MOSFETs rather than diodes to avoid the turn on voltage drop of diodes which can be significant for low output voltage power supplies. The transistors are biased to conduct from source-to-drain (for an N-channel power MOSFET) when a diode would have been conducting from anode to cathode, and conversely, are gated to block voltage from drain-to-source when a diode would have been blocking from cathode to anode.
In these synchronous rectifier circuits, the gate signals to the transistors must be synchronized as close as possible to the inflection points of the output current, which inflection points correspond to the zero crossings of the squarewave output voltage. The gate signals can be self-driven, i.e., the gate signal can be tied directly to the circuit, or controlled-synchronized, i.e., the synchronizing signal is derived from some point in the circuit and fed to the MOSFET gate driver. Typically, the synchronous rectifiers require auxiliary windings or additional connections to provide information to the synchronous rectifier circuit. The auxiliary winding or additional connection adds cost to the synchronous rectifier circuit solution.
Hence, a need exists for a synchronous rectifier that has a wide band of frequency operation, and maintains a minimum number of external pin connections. It would be of further advantage for the synchronous rectifier to be integrated and have reduced costs.